PLN CDR draft: Issue 2
1. Issue 2 (PRINT-READ consistency)
1.1. Description
The Lisp reader uses find-package when reading a symbol, which is affected by the
local nicknames of the current package. That means that to maintain print-read
consistency when printing a symbol, a good package prefix must be used - such that
calling find-package on it in the current package returns the home package of
the symbol.
There are several situations to consider:
The symbol is apparently uninterned.
In this case it is printed without the package prefix, using the uninterned symbol syntax
#:.The symbol is accessible in the current package.
In this case it is printed without any package prefix.
The name or one of the global nicknames of the home package of the symbol is not shadowed by any local nickname defined in the current package.
In this case that name or global nickname might be used as the package prefix.
There exists a local nickname defined in the current package for the home package of the symbol.
In this case that local nickname might be used as the package prefix.
Both the name and all global nicknames of the home package of the symbol are shadowed by local nicknames of the current package, and there is no local nickname defined in the current package for the home package.
It is not clear how the symbol is printed, see PROPOSALS.
1.2. Examples
(defpackage #:foo (:use) (:export #:+)) (defpackage #:bar (:use #:cl) (:local-nicknames (#:foo #:cl))) (let ((*package* (find-package '#:bar))) (print 'foo:+)) ; >> FOO:+ (sbcl, ccl, ecl, acl, abcl, clasp, lispworks) ;; In the package #:BAR symbol FOO:+ refers to CL:+
(defpackage #:foo-a (:use) (:export #:quux)) (defpackage #:foo-b (:use) (:export #:quux)) (defpackage #:bar (:use) (:local-nicknames (#:foo-a #:foo-b) (#:foo-b #:foo-a))) (let ((*package* (find-package '#:bar))) (print 'foo-a:quux)) ; >> FOO-B:QUUX (sbcl, ccl, abcl, lispworks) ; >> FOO-A:QUUX (ecl, acl, clasp) ;; In the package #:BAR symbol FOO-A:QUUX refers to FOO-B:QUUX
1.3. Current behavior
sbcl, ccl, abcl, lispworks: When possible, a local nickname is used as a package prefix.
ecl, acl, clasp: Local nicknames are never used as a package prefix.
1.4. Proposal SHARPSIGN-DOT
In the 5th case the symbol is printed using the #. syntax:
#.(cl:let ((cl:*package* (cl:find-package "KEYWORD"))) (cl:find-symbol "BAR" "FOO")) ;; or #.(cl:let ((cl:*package* (cl:find-package "KEYWORD"))) (cl:intern "BAR" "FOO"))
If *read-eval* is false and *print-readably* is true, an error of type
print-not-readable is signaled.
1.4.1. Note
Since #:KEYWORD cannot be used as a local nickname, and no local nicknames
can be defined in the #:KEYWORD package, this expression is guaranteed to
evaluate to the symbol in the correct package.
1.5. Proposal SHARPSIGN-COLON
In the 5th case the symbol is printed using the extended #: syntax:
#:(package name) #::(package name)
Shinmera's idea.
1.6. Proposal SHARPSIGN-BACKQUOTE
In the 5th case the symbol is printed using the new #` syntax for reading an
expression ignoring local nicknames in the current package:
#`foo:bar #`foo::bar
It can be implemented roughly as follows:
(defun |#`-reader| (stream subchar arg) (declare (ignore subchar arg)) (let* ((current-package *package*) (local-nicknames (package-local-nicknames current-package))) (loop for (nick . package) in local-nicknames do (remove-package-local-nickname nick current-package)) (unwind-protect (read stream t nil t) (loop for (nick . package) in local-nicknames do (add-package-local-nickname nick package current-package))))) (set-dispatch-macro-character #\# #\` #'|#`-reader|)
It is implementation-dependent whether local nicknames are actually removed from the current package or not.
1.7. Proposal PRINT-UNREADABLY
In the 5th case the symbol is printed unreadably using the #< syntax:
#<SYMBOL IN THE SHADOWED PACKAGE FOO:BAR> #<SYMBOL IN THE SHADOWED PACKAGE FOO::BAR>
(Specifics are implementation-dependent.)
If *print-readably* is true, an error of type print-not-readable is signaled.
1.8. Proposal THREE-FOUR-PACKAGE-MARKERS
In the 5th case the symbol is printed using the extended symbol token syntax:
foo:::bar ; same as (cl:find-symbol "BAR" "FOO") in the #:KEYWORD package foo::::bar ; same as (cl:intern "BAR" "FOO") in #:KEYWORD package